Letter pubs.acs.org/OrgLett
LDA-Mediated Synthesis of Triarylmethanes by Arylation of Diarylmethanes with Fluoroarenes at Room Temperature Xinfei Ji, Tao Huang, Wei Wu, Fang Liang, and Song Cao* Shanghai Key Laboratory of Chemical Biology, School of Pharmacy, East China University of Science and Technology (ECUST), Shanghai 200237, China S Supporting Information *
ABSTRACT: A practical and convenient approach for the secondary C(sp3)−H arylation of diarylmethanes with various fluoroarenes is described. The reaction proceeds smoothly in the presence of LDA (lithium diisopropylamide) at room temperature and affords triarylmethanes in moderate to high yields.
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practical methods for the synthesis of triarylmethanes is an important goal of organic synthesis. The cleavage and functionalization of carbon−fluorine bonds have attracted considerable attention from many chemists because of their important application in practical organic synthesis.6 The replacement of fluorine atom on the electron-deficient fluoroaromatics via nucleophilic aromatic substitution (SNAr) with nucleophiles is a traditional method for the functionalization of aryl fluorides.7 However, aryl fluorides with an electron-donating group would deactivate the SNAr reaction, and rather higher reaction temperature and prolonged reaction time are required to ensure a smooth reaction.8 The transition-metal-catalyzed cross-coupling of fluoroarenes with organometallics such as Grignard, organozinc reagents, and organoboron compounds,9 which involve the activation of the C−F bond, has become a powerful tool for the functionalization of fluoroarenes. However, the methodology suffers from the use of toxic nickel catalyst, expensive palladium catalyst, and complicated ligands.10 Defluorination of fluoroarenes via arynes in the presence of strong base, although known as a simple alternative for the cleavage of carbon−fluorine bonds, has not attracted enough attention in fluorine and organic chemistry.11 To date, only a few limited reports have been found in the literature.12 The obvious drawback of this method is the use of very low reaction temperatures, typically −78 °C, thus limiting the scope and generality of the protocol.13 In continuation of our interest in the cleavage and functionalization of carbon− fluorine bonds,14 we herein report an efficient and mild method for the synthesis of triarylmethanes by arylation of diarylmethanes with fluoroarenes in the presence of LDA (lithium diisopropylamide) at room temperature (Scheme 1, f).
riarylmethanes (TAMs) are often found as structural frameworks in pharmaceuticals and polymers, especially in leuco dyes due to their special photochemical and photophysical properties.1 Because of their importance in organic synthesis, considerable attention has been devoted to the development of efficient methods for the synthesis of triarylmethane derivatives.2 The traditional method for the synthesis of triarylmethane derivatives is Friedel−Crafts arylation of diarylmethanols in the presence of different Lewis acids.3 In recent years, a number of novel approaches toward triarylmethanes have been reported (Scheme 1, a−e).4 Scheme 1. Recent Methods for the Synthesis of Triarylmethanes
Although these reported methodologies are facile and straightforward, the limitations of these approaches include the use of an expensive ligand, an expensive palladium salt, the limited diversity of the starting materials, and harsh reaction conditions with regard to temperature and acidity.5 Hence, the development of environmentally benign and © XXXX American Chemical Society
Received: September 9, 2015
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DOI: 10.1021/acs.orglett.5b02602 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Initially, diphenylmethane 1a and fluorobenzene 2a were used as model reactants for the optimization of the arylation reaction conditions, and the results are shown in Table 1.
Scheme 2. Reactions of 1a with Various Substituted Fluorobenzenes 2a−la,b
Table 1. Optimization of the Reaction Conditionsa,b
a
entry
2a (equiv)
base (equiv)
solvent
yieldb (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.5 2.0 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5
KHMDS (3.0) LiHMDS (3.0) NaHMDS (3.0) t-BuOK (3.0) Cs2CO3 (3.0) t-BuOLi (3.0) t-BuLi (3.0) LDA (3.0) LDA (3.0) LDA (3.0) LDA (2.5) LDA (2.0) LDA (1.5) LDA (3.0) LDA (3.0) LDA (3.0) LDA (3.0) LDA (3.0)
THF THF THF THF THF THF THF THF THF THF THF THF THF Et2O hexane DMSO NMP DMF
0 0 0 0 0 0 0 64 93 89 86 61 35 47 36 0 0 0
a
Reaction conditions: 1a (1.0 mmol), 2a−e,g−j (1.5 mmol), 2f (3.0 mmol), 2k (1.2 mmol), LDA (3.0 mmol), THF (5 mL), rt, 1−4 h, Ar. b Isolated yield. cGC−MS yield.
b
Reagents and conditions: 1a (1.0 mmol), solvent (5 mL), 4 h. Yields were determined by GC analysis and based on 1a.
electron-rich fluoroarenes might proceed via aryne intermediates. To further confirm this hypothesis, two additional experiments were performed. When the highly electron-rich aryl fluoride such as 2-fluoro-1,4-dimethoxybenzene was used as substrate, the reaction also proceeded smoothly and no regioisomer was observed (3af). However, when 2-fluoro-1,3dimethylbenzene was used as substrate, no desired product was formed (3ae). These experiments clearly indicated that the arynes were generated in situ from the electron-rich fluoroarenes with LDA in the first step of the reaction. The fluoroarene possessing mildly electron-withdrawing group gave the desired products in lower yield than those with electron-donating substituents, and a mixture of paraand meta-isomers was obtained (3ah). However, when fluoroarene having two mildly electron-withdrawing groups was subjected to the optimized reaction conditions, only a moderate yield of product was obtained (3ai) and no regioisomer was detected. Much to our delight, fluoroarenes bearing a strong electron-withdrawing group (2j) and fluoropyridine (2k and 2l) afforded the arylated products in high yields without the formation of other isomers (3aj, 3ak, and 3al). On the basis of these experimental results, we assumed that the reaction of diphenylmethane 1a with electron-poor aryl fluorides might proceed via a classical SNAr pathway. Subsequently, the reactions of fluorobenzene 2a with various diarylmethanes were investigated (Scheme 3). Despite the inertness of C−F bond of fluorobenzene, most of the reactions proceeded smoothly at room temperature. Generally,
Among several bases tested, LDA turned out to be the only suitable base for the reaction, while other bases were totally ineffective and no product could be detected (entries 1−8). When the amount of fluorobenzene 2a was increased to 1.5 equiv, diphenylmethane 1a was successfully converted to the corresponding arylated product 3aa in excellent yield (entry 9). Further increasing the amount of 2a resulted in lower yield (entry 10). Moreover, the amount of LDA added has an important influence on the reaction yield. When the amount of LDA was reduced from 3.0 to 1.5 equiv of 1a, the yield decreased sharply and only 35% of 3aa was obtained (entries 11−13). Further screening of different solvents disclosed that THF proved to be effective for the reaction and provided the best results, while no reaction occurred in aprotic polar solvents such as DMSO, NMP, and DMF (entries 9 and 14− 18). To explore the substrate scope, we examined the reaction of a range of substituted fluorobenzenes with 1a under the optimized reaction conditions (Table 1, entry 9). As shown in Scheme 2, in most cases, fluoroarenes 2a−l proceeded smoothly and afforded the arylation products 3aa−al in moderate to good yields. The fluoroarenes bearing electronwithdrawing groups or an electron-neutral group (2a) gave higher yields than those containing electron-donating groups on the aromatic rings (2j versus 2g). Notably, fluoroarenes containing electron-donating groups provided a mixture of regioisomers (3ab,ac,ad,ag; the structures of the ortho-, meta-, and para-isomers were determined by their 1H NMR spectra and the literature). These results implied that the reaction of B
DOI: 10.1021/acs.orglett.5b02602 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Scheme 3. Reactions of 2a with Various Diarylmethanes 1b−ha,b
Scheme 5. Reactions of 9H-Fluorene 1i with Substituted Fluorobenzenesa,b
a
Reaction conditions: 1b−h (1.0 mmol), 2a (1.5 mmol), LDA (3.0 mmol), THF (5 mL), rt, 1−4 h, Ar. bIsolated yield.
the electron-poor diarylmethanes gave good yields whereas the electron-rich diarylmethanes provided lower yield (3ba). To evaluate the scope of the reaction further, several diarylmethanes, 9H-fluorene 1i and 9H-xanthene 1j were reacted with different fluoroarenes (Schemes 4−6). Generally,
a
Reaction conditions: 1i (1.0 mmol), 2a, 2b, 2g, 2j, 2k, 2m (1.5 mmol), LDA (3.0 mmol), THF (5 mL), rt, 1−4 h, Ar. bIsolated yield.
Scheme 6. Reactions of 9H-Xanthene 1j with Substituted Fluorobenzenesa,b
Scheme 4. Reactions of Diarylmethanes with Substituted Fluorobenzenesa,b
a
Reaction conditions: 1j (1.0 mmol), 2a,b,m (1.5 mmol), and 2f (3.0 mmol), LDA (3.0 mmol), THF (5 mL), rt, 1−4 h, Ar. bIsolated yield.
C(sp3)−H arylation of diarylmethanes, 9H-fluorene, and 9Hxanthene with various readily available fluoroarenes. This direct arylation was promoted only by LDA. More importantly, with this protocol, both unreactive C−F bonds such as the C−F bond in 2f and 2g and reactive C−F bonds such as the C−F bond in 2j could be readily cleaved in the presence of LDA at room temperature within short reaction times. The arylation of diphenylmethanes with electron-rich aryl fluorides proceeded via aryne intermediates, whereas electron-poor fluoroarenes proceed via a classical SNAr pathway. Furthermore, the arylation process exhibited good substrate scope and functional group tolerance, and a variety of triarylmethane derivatives were obtained in moderate to high yields. Further efforts to extend the applications of this method are underway in our laboratory.
a Reaction conditions: 1c,d,f−h (1.0 mmol); 2g,j−l (1.5 mmol), 2f (3.0 mmol), LDA (3.0 mmol), THF (5 mL), rt, 1−4 h, Ar. bIsolated yield.
the reaction proceeded smoothly at room temperature to yield arylated products, and the observed trend was the same as that seen for above-mentioned fluoroarenes and diarylmethanes: electron-rich fluoroarenes tended to afford two regioisomers (3cg,dg,ib,ig,jb), whereas electron-poor fluoroarenes afforded only one isomer (3cj,dk,fk,gk,hl,ij,ik,im,jm). In summary, we have developed a facile and transitionmetal-free synthesis of triarylmethanes by the secondary C
DOI: 10.1021/acs.orglett.5b02602 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.5b02602. Experimental details and spectroscopic data for compounds 3 (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Tel: +86-21-64253452. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for financial support from the National Natural Science Foundation of China (Grant Nos. 21472043 and 21272070).
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REFERENCES
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DOI: 10.1021/acs.orglett.5b02602 Org. Lett. XXXX, XXX, XXX−XXX
